Insulated bushing



Feb. 16, 1954 J. S. JOHNSON FI'AL INSULATED BUSHING Filed Aug. 19, 1950lIIIII/I/Il/III/IIlI/l WITNESSES: 4 INVENTORS Z John S. Johnson,ChurlesC. Sferretf 0nd Josepl'gae R Gerber A ORNEY Patented Feb. 16, 1954INSULATED BUSHING John S. Johnson,

Charles C. Sterrett, and

Josephine R. Garber, Pittsburgh, Pa., assignors to Westinghouse.Pittsburgh, Pa., a

Electric Corporation, East corporation of Pennsylvania ApplicationAugust 19, 1950, Serial No. 180,352

11 Claims. (Cl. 174152) This inventionrelates to improved insulatedbushings suitable for use with electrical currents having a high voltageand a high amperage.

At the present time the electrical industry employs large numbers oflead bushings in generators,-switchgear and similar apparatus. For highvoltage use the bushings are usually constructed with a centralconducting stud and an exterior spaced porcelain sleeve sealed to theends'of the stud. The space between the porcelain sleeve and the stud isfilled with oil in order to-provide dielectric strength and to dissipateheat developed by the passage of electrical current through the stud. Inmany cases the space between the porcelain sleeve and the stud ispartially filled with a wrapping of paper. but oil is necessary toinsure proper operation of the-bushings in service. The oil fillingenables bushingsto meet certain standards of impulse strength andwithstand surges. Furthermore, it is necessary that bushings notgenerate radio interfering energy.

As the output of generators has gone up particuarly with considerableincreases in amperage, the size of oil-filled lead bushings must beincreased correspondingly. The problem of providing an oil-tight sealbetween the ends of the porcelain sleeve and the stud becomesincreasingly more difiicult as the size of the bushing increases. Thisis due to the fact that during firing porcelain shrinks non-uniformlyand erratio changes in dimensions occur, such changes increasing withsize. Other disadvantages, such as fire hazard, cost and increased bulkof the apparatus, necessitated by the larger sized bushings has made itless and less desirable to use an oil-filled bushing. However, previousto the present invention; an oil-filled bushing construction was theonly one known or available that would satisfactorily meet mostrequirements.

The object of the present invention is to provide an insulatedelectrical bushing of relatively small size that will handle electricalcurrents with adequate efiiciency without the use of a liquid insulatingmedium.

A further object of the invention is to provide a bushing'comprising astud and a spaced porcelain sleeve in which the space is substantiallycompletely filled with a mica tape and a solid thermoset resin.

A still further object of the invention is to provide a. process forpreparing a bushing in which the space between the porcelain sleeve andthe stud is substantially completely filled with mica tape wrapping anda thermoset resin.

Other objects of the invention will in part be obvious, and will in partappear hereinafter;

For a better understanding of the nature and objects of the invention,referenc should be had to the following detailed description anddrawing, in which:

Figure 1 is a view in elevation; and

Fig. 2 is a view in elevation, partly in section, showing the steps ofpreparing an insulated bushing in accordance with the invention.

We have discovered that highly eflicient and unusually satisfactoryelectrical bushings capable of carrying a load that otherwise can onlybe safely carried by a much larger oil-filled bushing, may be preparedby filling the space between the conducting stud and a spaced porcelainsleeve thereon with a mica tape and a completely reactive thermosettingresin impregnant applied to the mica flakes.

In accordance with the present invention, the portion of the studsurrounded by the porcelain sleeve is wrapped with a mica tape or sheetin order to fill most of the space that would normally be presentbetween the porcelain sleeve and the stud. The mica tape may be of anysuitable Width and may be applied in a sufficient number of layers toprovide the required size or diameter to substantially fill the bore ofthe porcelain sleeve to be applied thereto. For most purposes, from 4 to50 or more layers of mica tape, each layer of an average thickness of '7to 10 mils, are sufiicient.

It may be desirable to wrap one or more metallic condenser foils betweenthe layers of mica tape to enable the potential stresses to be graded sothat the insulation will function be ter in service.

For the purpose of th present invention, conventional mica tapematerials are not suitable. Resin binders for mica flakes which havebeen employed heretofore, namely shellac, alkyd resins and asphalt,result in a tape that will not satisfactorily impregnate with thecompletely reactive resin compositions that have been found to producesatisfactory bushings. Any gas or air pockets in the mica tape will giverise to corona and be otherwise unsatisfactory. For the satisfactorypractice of the present invention it is necessary to employ a mica tapein which the mica flakes are bonded to each other by a liquid binder. Wehave found particularly satisfactory a mica tape embodying a liquidresinous binder having a viscosity of 25 to 10,000 poises at 25 C. Thebinder may be composed entirely of one or more liquid resinous polymershaving the desired viscosity, or it may be composed of a liquid mixtureof a polymer and a liquid plasticizer, which mixture has the desiredviscosity. The liquid resinous polymers are stable and relativelynonvolatile and non-depolymerizing at room temperature and at alloperating temperatures of the bushings embodying them so that compositeisfactory results. The following table sets forth the viscosities of anumber of compositions produced in accordance with the invention.

mica insulations prepared therewith may be handled, stored and usedwithout significant loss or change in properties. Application Serial No.161,231, filed May 10, 1950, and assigned to the assignee of the presentinvention, discloses liquid polymer binders and composite mica tapesprepared therewith suitable for use in practicing the present invention.Examples of such binders and tapes prepared therewith are set forth inthe following table:

TABLE I Approxi' Percent Polymer 'gi g Bond 111 Tape WeightPolymethylmethacrylate l, 000 10. Poly-a-methyl styrene. 4, 000 16.4.Polystyrene 1, 500 11.2. Coumarone-indene 1,000 6.Polyalphamethylstyrene 3, 000 3 to 25 Polymers of styrene and coumaronemixtures. 6, 000 8.

Reference should be had to that application, for example of otherbinders and methods of preparing mica tapes therewith.

Another suitable tape that may be employed in practicing the inventionis one prepared with a binder composed of from to 75% by weight ofpolystyrene of a molecular weight of between 30,000 and 250,000, andfrom 85% to 25% by weight of a plasticizer selected from the groupconsisting of liquid polycyclic hydrocarbons boiling above 270 C. andhaving a viscosity of from 0.5 to 5 poises at 25 0., selected from thegroup consisting of terphenyls hydrogenated at least 40%, monoamylnaphthalene, diamyl naphthalene and polyamyl naphthalene, and mixturesthereof. The hydrogenated terphenyl may be produced by pyroliticdecomposition of benzene as set forth in Patent No. 2,364,719, or it maybe produced by reacting benzene and cyclohexyl bromide in the presenceof aluminum chloride to produce dicyclohexyl benzene. A specific productthat has been found satisfactory for the purpose of the invention is anisomeric mixture of ortho, meta, and para terphenyls hydrogenatedapproximately 55%, which mixture has a distillation range of from 345 C.to 425 C. and a viscosity of approximately 0.8 poise at 25 C.

Monoisoamyl naphthalene boiling in the range of 280 C. to 330 C., diamylnaphthalene boiling in the range of 330 C. to 365 C. and having aviscosity of 0.9 poise at 25 C., and polyamyl naphthalene boiling in therange of 355 C. to 400 C. and having a viscosity of 5.5 poises at 25 C.,individually or in admixture may be employed in combination Withpolystyrene with sat- The mica tape to be employed in making bushingspreferably has a backing comprising a sheetmaterial to enable the micaflakes to be handled easier and wrapped more tightly about bushingstuds. If desired, the mica flakes may be applied as a layer to thesheet backing, the liquid binder applied and then a to sheetsuperimposed to produce the strongest tapes.

The pliable sheet base material, for instance,-

may be rice paper or supercalendered rope paper or other paper. Suchpapers are available in thicknesses as low as of the order of 1 mil(0.001"). Furthermore, the sheet base material may comprise othermaterials such as glass fiber cloth, asbestos cloth, asbestos paper orcotton cloth. The asbestos paper may comprise ordinary milled asbestosadmixed with small amounts of cellulosic fibers or it may be formed fromasbestos that has been ball-milled to an extremely;

fine subdivision of the asbestos fibers, often called microfineasbestos. Synthetic resins either in the form of films or Woven fabrics,such, for example, as nylon cloth or nylon film, may be employed as thesheet base material. Woven fabric from fibers prepared from linearpclymethyleneterephthalate, or films from the same polymer are anexcellent backing material. The preparation of these terephthalatepolymers is set forth in Whinfield et al. U. S. Patent 2,465,319. Sheetsof other synthetic resins, such as cellulose acetate 1 butyrate orpolyethylene may be employed as a base for the composite micainsulation. It is not necessary that both of the two base sheets appliedon either side of the layer of the mica flakes be of the same materfal.

bestos and glass fiber fabric. Glass fiber cloth or asbestos paper maybe employed respectively for the top and the bottom of the tape.

The mica flakes may be applied to the sheet backing by hand or in aconventional mica laying mach ne to any desired thickness of a layertheretion of a viscosity of about 10 poises and less, such solutionhaving from 5% to by weight of the polymer dissolved therein, is appliedtothe layer of the mica flakes in a quantity sulficient to wet them. Thewetted layer of mica flakes is rolled to disperse the solution betweenthe mica flakes and to distribute it larly Wetted with the bindersolution as required. A top sheet similar to the base or backing sheetmay be applied, and the composite.-

Thus, the composite mica msulation may comprise a backing of paperand asuperposed layer of asbestos paper, or asuniformly and to wet the sheetbase material. Additional layers of mica flakes may be applied andsimi-.

sheetv so produced rolled to insure good distribution of the solution ofthe binder; The sheet so produced is then heated to evaporate thevolatile solvent, leaving only the liquid binder uniting the sheets andthe mica flakes. If necessary, the sheet may be slit into suitable tapewidths. The liquid binders have been found to wet the mica and to adhereto them to enable a relatively strong insulation to be secured. Thecomposed insulation may be handled, cut, wrapped and processed withoutexcessive separation or.

loss of the mica flakes. The liquid binders employed have been found notto impair the sheet backing material, such as paper, either in strengthor in flexibility. The liquid binders will not evapcrate or harden onstorage.

Referring to Fig. l of the drawing, there is illustrated the step ofapplying the micatape toa stud of an insulating bushing. The stud l0,composed of copper or other good conducting metal, is provided withthreads I2 and M at its opposite ends. The copper metal forming stud lmay inhibit polymerization of certain unsaturated compositions if leftbare, so it is desirable to silver plate the stud or to coat it with avarnish such as a wire enamel, before processing it. At the one end, acollar I6 closely fitting the outside diameter of the bushing IE1 isheld in position by a spanner nut l8 with engageable slots 20 whichnutis threadedly engaged with the thread M. The collar 16 is adapted tofit closely one end of a porcelain sleeve to be employed with the studit. If required, threaded apertures 22 may be provided in the collar Itfor introducing a resin as will be set forth subsequently. Threaded caps(not shown) may be employed to close the apertures 22. A plurality oflayers 24 of mica tape 26 are wrapped along the diameter of that portionof the stud to be enclosed by a subsequently applied porcelain sleeve.The tape 26 comprises mica flakes 28 and the sheet backing 29 unitedwith the liquid binder as set forth herenbefore.

After a suflicient number of layers 24 of the mica tape have beenwrapped on the stud ID, the stud with the applied tape is heated andevacuated to remove gases and moisture therefrom in order to enable theimpregnation of the tape with certain resinous compositions to becarried out to best advantage. Several alternative procedures are avalable in impregnating and assembling the bushing.

We have found that the procedure that appears most practical and hasgiven very satisfactory results in service is the one illustrated inFig. 2 of the drawing. In this preferred procedure a porcelain sleeve 30is positioned upon the stud H) with its wrapping 24 of mica tape. Asillustrated, the lower end 32 of the porcelain sleeve 30 is insubstantially liquid tight engagement with the collar [6. The threadedapertures 22 are not present or are capped in this procedure. Ifnecessary a flexible gasket or the like may be interposed between thecollar I6 and the lower end 32 of the bushing. The bushing 30 has aninternal bore 34 slightly larger than the wrapping 24 of mica flakes,thereby providing a slight space 36 for clearance. The exposed surfacesof the assembly of the stud l0 and the porcelain sleeve 30 are coatedwith a readily removable protective material 38 to prevent the adhesionof subsequently applied resins to such threaded surfaces. As will beevident from Fig. 2 of the drawing, the protective material 38 isapplied.

to the exterior portion of the porcelain sleeve 30, the threads l2 and[4 of the stud, the interior bore 4|. 0:. the stud, and the upper end 42or the porcelainsleeve. We have found that polyvinyl alcohol may besprayed or brushed on the assembly to function as a protective compound.Numerous other strippable compounds may be applied, such, for example,as ethyl cellulose, cellulose acetate, asphalt and the like may beemployed as a protective coating. Care should be taken, however, thatnone of the protective material 38 enters the space 36 or coats the bore34 or the mica tape layer 24.

The partly assembled and placed upright on a supporting fixture 45provided with legs48, as shown in Fig. 2 of the drawing, disposed withina tank provided with a sealable cover 52. Tank 50 conduit 54 which maybe connected to a suitable source of vacuum and a source of gas underpressure to enable the bushing assembly to be evacuated or otherwisetreated. At the lower portion of the tank 50 is attached a conduit 56provided with a valve 58 enabling an impregnating composition. to beintroduced into a tank or to be withdrawn therefrom.

The bushing assembly shown in Fig. 2 of the drawing may have beenpreviously heated in an ovento a temperature not exceeding 175 0., andpreferably between C. and C., so'as to drive off any moisture orvolatiles from the mica tape wrapping 24. However, the tank 50 may beprovided with a suitable heater for this purpose if desired. In eithercase after heating, and when placed within the tank 50, the bushingassembly is evacuated to an absolute pressure of not over 5 inches ofmercury in order to withdraw any gases from the interstices of thewrapping 24. Thereafter a liquid completely reactive thermosettableresinous composition 60 is introduced through conduit 56 into the tank56 to a level above the upper end 42 of the porcelain sleeve so that thecomposition 60 can enter the space 36. After the space 36 has beenfilled, a gas under pressure may be introduced through the conduit 54 toforce the composition 653 into the wrapping 24 of the mica tape tosubstantially completely fill the voids and interstices thereof.Thereafter the composition 60 may be drained from the tank 50. Theimpregnated, partly assembled bushing may be heated in tank 50 orremoved therefrom for heating in an oven to a temperature sufficient tocause the completely reactive composition to polymerize to a solidresin.

While a tape comprising mica flakes bonded with a liquid resinous bindergives excellent results in practicing the invention, we have found thatmica flakes bonded with a volatile organic liquid may b employedsuccessfully. Such volatile liquids form a temporary bond that holds themica flakes and the backing together as a tape to enable the tape to bewrapped on the stud. Once wrapped, there is no need for a binder in thetape and it is permitted to evaporate either naturally or duringheating. Examples of suitable organic liquids of a non-resinous natureare amyl naphthalene, benzene, toluene, Xylene, ethyl lactate, butanoland higher alcohols, and naphtha. In preparing a tape, mica flakes laidon a sheet of paper are sprayed with the organic liquid and immediatelywound into a tape roll whileso wetted. The tape roll is put into a canor other vapor-tight container to prevent escape of the organic liquidbinder. When required for taping a stud, a roll of tape is removed fromthe container and promptly wrapped on the stud. The tape is surprisinglystrong and holds together well. Once the temporary organic liquid bindercoated bushing is is provided with a.

aeeaeor is removed from the wound tape by heating and evacuation,impregnation with a completely reactive resin composition is easilyeffected.

Completely-reactive compositions suitable for use in the process mayinclude any fluid polymerizable composition, ordinarily with a catalyst,that, upon being heated to a predetermined temperature, polymerizesentirely into a thermoset, relatively-hard resinous body. The fluidresinous composition may comprisev a single polymerizable monomericcomponent such, for example, as diallyl phthalate, diallyl succinate,diallyl maleate, diallyl adipate, allyl alcohol, methallyl acrylate,diallyl ether, allyl acrylate, and allyl crotonate. It will be notedthat such monomeric components comprise at least two unsaturated groupscapable of vinyl-type addition polymerization.

Good results have been secured with only one Numerous othercompletely-reactive multicomponent polymerizable compositions are knownto the art. Such compositions include, in many cases, an unsaturatedresinous component-particularly an unsaturated polyester resin-and anunsaturated polymerizable liquid monomer. Particularly good results havebeen secured by employing as the polyester resin the reaction product ofan ethylenic dicarboxylic acid or anhydride thereof such, for example,as maleic acid, fumaric acid, maleic anhydride, monochloromaleic acid,itaconic acid, itaconic anhydride, citraconic acid and citraconicanhydride. The unsaturated dicarboxylic acid or anhydride or mixturesthereof are reacted with a substantially molar equivalent of apolyhydric alcohol such as ethylene glycol, glycerol, propylene glycol,diethylene glycol, or pentaerythritol or mixtures thereof. Castor oilhas been employed successfully in reactions with maleic anhydride, andthe resultant castor oil maleate ester admixed with a polymerizableunsaturated monomer, for example, monostyrene, in the proportions offrom 10 to 95 parts by weight of the monostyrene and from 90 to parts byweight of the ester. In the preparation of the unsaturated alkyd esters,an ethylenically unsaturated alpha-beta dicarboxylic acid or anhydridemay be replaced with up to 95% of the weight thereof by a saturatedaliphatic di carboxylic acid or aryl dicarboxylic acid or anhydride,such, for example, as succinic acid, adipic acid, sebacic acid, phthalicacid, phthalic anhydride or the like. Also, mixtures of polyhydricalcohols may be employed. In some instances, epoxides have been employedin lieu of glycols, particularly in reactions with dicarboxylic acidsinstead of their anhydrides.

The unsaturated alkyd esters may be dissolved in a liquid unsaturatedmonomer having the group HzC:C Suitable liquid unsaturated polymerizablemonomers are: monostyrene, alphamethylstyrene, 2,4-dichlorostyrene,paramethyl styrene, vinyl acetate, methyl methacrylate, ethyl acrylate,diallyl phthalate, diallyl succinate, diallyl maleate, allyl alcohol,methallyl alcohol, acrylonitrile, methyl vinyl ketone, diallyl ether,vinylidene chloride, butyl methacrylate, allyl acrylate, allylcrotonate, 1,3-chloroprene, and divinyl benzene, as well as mixtures oftwo or more of any oithese monomers.

An excellent completely reactive composition is one composed'of asolution in from to 50 parts of arylalkene polymerizable liquid monomer,such as monostyrene, of from 10 to 50 parts by weight of the alkydreaction products of (A) an unsaturated acidic compound from the groupconsisting of maleic acid, maleic anhydride, fumaric acid, citroconicacid and citroconic anhydride in admixture with one or more saturatedstraight chain dicarboxylic acids having the carboxyl groups disposed atthe end of the straight chain, the chain having from 2 to 10non-carboxyl carbon atoms and not other reactive groups, and (B) a molarequivalent within +10% of an aliphatic saturated glycol having no otherreactive group than the hydroxyl groups. The proportion of theunsaturated acidic compound in the (A) mixture of acids should bebetween 5% and 50% of the weight of the mixture. Suitable saturateddicarboxylic acids are adipic acid, sebacic acid, azelaic acid, subericacid, succinic acid, decamethylene dicarboxylic acid and diglycolic acidand mixtures thereof. With the longer chain saturated dicarboxylicacids, as, for example, sebacic acid, the proportion of maleicanhydride, for example, may be higher than if the saturated acid wereall succinic acid, to produce ultimate cured products of similar degreesof hardness. Examples of suitable (B) glycols for reaction with (A) themixture of saturated and unsaturated acids are ethylene glycol,propylene glycol, dicthylene glycol, 1,5-pentanediol and triethyleneglycol. Mixtures of the glycols are suitable for producing the reactionproduct. The reaction of the (A) acidic compounds and (B) the glycolsmay be carried out by heating in a reaction vessel at temperatures offrom C. to 250 C. for from 24 hours to 2 hours to a low acid number ofbelow 60.

The following are specific examples of the preparation of theunsaturated alkyd reaction products to be dissolved in the vinyl arylmonomer:

Example I A mixture of 44 mole percent of adipic acid and 6 molepercentof iumaric acid was combined with 50 mole percent of propylene glycoland reacted with CO2 sparging for about 4 hours at C. in a closedreaction vessel after which the temperature was raised to 220 C. over a4-hour period and the reaction was continued at 220 C. for 8 hours. Asyrupy polyester resin was produced.

Another composition comprised the reaction product of 10 mole percent ofmaleic anhydride, 40 mole percent of adipic acid, and 50 mole percent ofdiethylene glycol.

Example II A reaction product was prepared by reacting 30 mole percentof sebacic acid, 20 mole percent of maleic anhydride, and 50 molepercent of di- The unsaturated esters or alkyd resins of these twoexamples so prepared are dissolved in a monomeric compound having thegroup H2C=C such as monostyrene or a simple substitution derivative ofmonostyrene, or a mixture of two or more monomers, as above described,to produce low viscosity, completely reactive solutions having presentfrom 10% to 50% by weight of the unsaturated ester. Particularly goodresults have been obtained by dissolving the unsaturated esters inmonostyrene to produce solutions containing from 35% to 85% by weight ofmonostyrene and the balance, 65% to 15% by weight, composed of theunsaturated esters. Such solutions are solvent reactive compositionsthat will polymerize completely upon heating in the presence of one ormore vinyl-type polymerization catalysts, such a benzoyl peroxide,lauroyl peroxide, methyl ethyl ketone peroxide, t-butyl hydroperoxide,ascaridol, tert-butyl perbenzoate, di-tbutyl diperphthalate, ozonides,peracids and similar catalysts, in an amount of from 0.1% to 2% andmore, by weight. The proportion of the catalysts obviously may bepresent in amounts difiering from these percentages. Accelerators suchas metallic driers, for example, cobalt naphthenate; tin salts such astin chloride; or azomethine or polyamine promoters, as set forth inPatent 2,450,552, may be added in small amounts not exceeding 2%, to thecompositions.

After the completely reactive composition impregnated into the space 36of the bushing assembly shown in Fig. 2 of the drawing has beenpolymerized, it will be found that the mica tape wrapping 24 and thespace is filled with the solid thermoset resin. Thereafter thestrippable protective coating 38 may be readily removed by immersing thebushing assembly in a suitable solvent for the composition. Thethermoset composition will not be aifected by conventional solvents. Thebushing assembly may be thereafter completed by screwing on a suitablecollar at the threaded portion 42 to abut the upper end 42 of theporcelain sleeve 30.

An alternative procedure for impregnating the mica tape wrapping 24 andto fill the space 36 with thermoset resin that has been found to beadvantageous is as follows: A porcelain sleeve 30 is slipped over thewrapping 24 until the lower end 32 of the bushing tightly abuts thecollar l6. Then a second collar similar to I 6 is applied to the otherend of the stud l and threadedly engaged with the threads H of the studIt! so as to seal the upper end 42 of the bushing. The assembled bushingcan then be heated, and evacuated through the threaded aperture 22 inthe collar It. The completely reactive composition, such as 60, may thenbe poured in through one of the apertures 22 to fill the space 36.Vacuum may be applied to the other aperture 22 while the resincomposition is being introduced, and when the space 36 is full, externalpressure may be applied through the aperture 22 to force the compositioninto the interstices of the tape Wrapping 24. The bushing can then beheat treated to a temperature suflicient to polymerize completelyreactive composition. It will be noted that this procedure does notrequire the application of the strippable protective composition ,38.This procedure is particularly advantageous for extremely large bushingswhich are dificult to handle and impregnate according to the procedureillustrated in Fig. 2 of the drawing.

Another procedural modification that has been found suitable is tosubject the mica tape Wrapped stud IU of Fig. 1, without applying theporcelain sleeve thereto, to successive heating, evacuation and resinimpregnation steps. The mica tape layer 24 willbe saturated with theliquid completely reactive composition. The mica tape layer is-thenwrapped with a layer of anv impervious sheet material, such. as apolyvinyl also.

hol sheeting, polytetrafiuoroethylene tape or synthetic rubber. Thistape wrapping prevents the loss or exudation of the liquid composition.The stud with this last-mentioned impervious wrapping is then placed inan oven and baked for a period of time sufficient to polymerize thethermosettable composition. Then the impervious sheet Wrapping may bestripped. Ordinarily the mica tape wrapping in this latter expedient isapplied in a quantity to approximate very closely to the dimensions ofthe bore 34 of the sleeve 30 to be applied thereover. When the sleeve 30is slipped onto the stud, very little air space is present. In order tofit the bore of the porcelain sleeve the polymerized resin impregnatedmica tape wrapped may be machined by turning in a lathe or grinding tosize. It is not necessary that the space he filled, though it may bedesirable to introduce a small amount of the liquid composition afterthe porcelain sleeve 30 has been put into place and to heat the bushingagain to polymerize this small quantity of subsequently appliedcomposition.

Suitable temperature for curing the completely reactive compositions maybe as low as C. and as high as 150 C., or slightly more. Heating for onehour at from C. to C., followed by heating for several hours at atemperature of C., will cure the reactive compositions of Examples I andII. The composition may be heated in any suitable manner includin theuse of a high frequency field.

Bushings constructed in accordance with the present invention willwithstand much higher temperatures in use than is possible in oil-filledbushings employed at the present time. The electrical properties of theimpregnated mica tape Wrapping of the present invention reduces thepossibility of corona taking place even at the highest operatingvoltages. Thus, bushings of a rating of 23 kilovolts and higher showedno corona on full voltage tests when constructed as disclosed herein. Wehave been able to greatly reduce the size of a bushing by employing theimpregnated mica tape wrapping disclosed herein in place of theconventional oil-filled structures previously required. The bushings ofthe present invention avoid the problem of oil leaks and the like whichhave been a disadvantage heretofore. Furthermore, the use of an oiloften results in the formation of sludges and the like that impair thefunctioning of the prior art bushings. The bushings of the presentinvention do not deteriorate in service due to temperature, atmosphericoxidation or other service conditions.

Since certain obvious changes may be made in the above procedure anddiiferent embodiments of the invention could be made without departingfrom the scope thereof, it is intended that all matter contained in theabove description and drawing shall be taken in connection with theaccompanying claims and not in a limiting sense.

We claim as our invention:

1. A bushing comprising, in combination, a stud, an insulating porcelainsleeve of a lesser length than the stud and disposed about the outsideof the stud, a space being present along the full length between theporcelain sleeve and the stud, a wrappin of a tape applied to the studto substantially fill the space for the entire length of the porcelainsleeve, th tape comprising mica flakes and a sheet backing, and a bodyof thermoset resinfilling the entire space and solidly impregnating thetape whereby no voids are present between the stud and the porcelainsleeve, the resin comprising essentially the product derived bypolymerizing a completely reactive, thermosettable compositioncomprising a liquid monomeric compound having the group H2C=C .2. Abushing comprising, in combination, a stud, an insulating porcelainsleeve of a lesser length than the stud and disposed about the outsideof the stud, a space being present along the full length between theporcelain sleeve and the stud, a wrapping of a tape applied to the studto substantially fill the space for the entire length of the porcelainsleeve, the tape comprising mica flakes and a sheet backing, and a bodyof thermoset resin filling the entire space and solidly impregnating thetape whereby no voids are present between the stud and the porcelainsleeve, the resin comprising essentially the product derived bypolymerizing a completely reactive, thermosettablecomposition comprisinga liquid mono meric compound having the group H2C=C and an unsaturatedpolyester dissolved in the liquid monomeric compound, the polyesterhaving C=C groups reactive with the monomeric compound to form anaddition polymer therewith.

3. A bushing comprising, in combination, a stud, an insulating porcelainsleeve of a lesser length than the stud and disposed about the outsideof the stud, a space being present along the full length between theporcelain sleeve and the stud. a wrapping of a tape applied to the studto substantially fill the space for the entire length of the porcelainsleeve, the tape comprising mica flakes and a sheet backing, and a bodyof thermoset resin filling the entire space and. solidly impre natingthe tape whereby no voids are present bet een the stud and the porcelainsleeve, the resin comprising essentially the product derived bypolymerizing a completely reactive, thermosettable compositioncomprising a liquid monomeric liquid arylalkene compound having an H2C=Cgroup and an unsaturated polyester dissolved in the liquid monomericcompound, the polyester having C=C groups reactive with the arylalkenemonomer to form an addition polymer therewith.

4. A bushing comprising, in combination, a stud, an insulating porcelainsleeve of a lesser length than the stud and disposed about the outsideof the stud, a space being present along the full length between theporcelain sleeve and the stud. a wrapping of a tape applied to the studto substantially fill the space for the full length of the porcelainsleeve, the tape comprising mica fiakes and a sheet backing, and a bodyof thermoset resin filling the entire space and solidly impregnating thetape whereby no voids are present between the stud and the porcelainsleeve, the resin comprising essentially the product derived bypolymerizing a completely reactive liquid polymerizable compositionconsists of (a) from 50% to 90% by weight of at least one monomericarylalkene compound and (b) from 50% to 10% by weight of an unsaturatedpolyester derived by reacting by heating. (A) a mixture of from 95% to50% by weight of saturated acidic compounds and from 5% to 50% by weightof ethylenically unsaturated acidic compounds, the saturated acidiccompounds composed of straight chain dicarboxylic acids having thecarboxyl groups at the ends of the chains, the chains having from 2 tocarbon atoms, and no other reactive groups than the carboxyl groups, andthe unsaturated acidic compounds being selected from the groupconsisting of alpha, beta-ethylenically unsaturated dicarboXyllQ acidsand an 12 hydrides thereof, and (B) a molar equivalent within ':10% ofthe mixture of acidic compounds of an aliphatic saturated glycol havingno other reactive groups than the hydroxyl groups.

5. A bushing comprising, in combination, a stud, an insulating porcelainsleeve of a lesser length than the stud and disposed about the outsideof the stud, a space being present along the fulllength between theporcelain sleeve and the stud, a wrapping of a tape applied to the studto substantially fill the space for the entire length of the porcelainsleeve, the tape comprising mica flakes and a sheet backing, and a bodyof thermoset resin filling the entire space and solidly impregnating thetape whereby no voids are present between the stud and the porcelainsleeve,.the resin comprising essentially the product derived bypolymerizing a completely reactive liquid polymerizable compositionconsists of (a) from 50% to by weight of monostyrene and (b) from 50% to10% by weight of the polyester reaction product derived by heating from6 to 10 mole percent of an acidic compound selected from the groupconsisting of fumaric acid and maleic anhydride, from 44 to 40 molepercent of adipic acid and 50 mole percent of propylene glycol.

6. A bushing comprising, in combination, a stud, an insulating porcelainsleeve of a lesser length than the stud and disposed about the outsideof the stud, a space being present along the full length between theporcelain sleeve and the stud, a wrapping of a tape applied to the studto substantially fill the space for the entire length of the porcelainsleeve, the tape comprising mica flakes and a sheet backing, and a bodyof thermoset resin fillin the entire space and solidly impregnating thetape whereby no voids are present between the stud and the porcelainsleeve, the resin comprising essentially the product derived bypolymerizing a completely reactive liquid polymerizable compositionconsists of (a) from 50% to 90% by weight of monostyrene and (b) from50% to 10% by weight of the polyester reaction product derived byheating from 25 to 35 mole percent of sebacic acid, from 15 to 25 molepercent of maleic anhydride and 50 mole percent of propylene glycol.

'7. A bushing comprising, in combination, a stud, an insulatingporcelain sleeve shorter than the stud, the porcelain sleeve disposedabout the stud, a space being present between the stud and the porcelainsleeve, a wrapping of mica tape applied to the portion of the studembraced by the porcelain sleeve to substantially fill the major portionof said space for the entire length of the porcelain sleeve, the micatape comprising mica flake and a sheet backing, and a thermoset resinsolidly impregnating the mica tape wrapping whereby no voids are presentin the tape wrapping, the thermoset resin comprising essentially theproduct derived by polymerizing a compound having at least two C=Cgroups capable of addition polymerization.

8. In the process of preparing a bushing comprising a stud and a boredporcelain sleeve shorter than the stud to be applied about the stud witha space between the stud and the bore of the porcelain sleeve, the stepscomprising wrapping with a mica tape that portion of the stud aboutwhich the porcelain sleeve is to be applied, the mica tape comprising afibrous sheet backing, a layer of mica flakes applied to the sheetbacking and a liquid binder uniting the mica flakes and the backing, themica tape wrapping being thick enough substantially to fill the spacebetween the sleeve and the stud heating the stud and applied mica tapewrapping to a temperature not exceeding 175 C., evacuating the stud andapplied mica tape wrapping to remove volatiles, impregnating theevacuated wrapping with a liquid, completely reactive, thermosettablecomposition comprising a monomeric compound having the group H2C=C theliquid composition penetrating readily through the mica tape and fillingall the interstices thereof, wrapping the impregnated mica tape with aflexible sheet of a material impervious to the fluid composition,applying heat to the stud with said last-mentioned wrapping thereon inorder to polymerize the composition to a thermoset solid, thereafterremoving the last-mentioned wrapping of impervious material, machining,if necessary, the solidly impregnated mica tape wrap-ping to a sizesmaller than the bore of the porcelain sleeve, placing the porcelainsleeve over the stud with its impregnated mica tape wrapping, thereafterfilling the space between the wrapped stud and the porcelain sleeve withthe liquid thermosettable composition and polymerizing the compositionto a solid.

9. In the process of preparing a bushing comprising a stud and a boredprocelain sleeve shorter than the stud to be applied about the stud witha spac between the stud and the bore of the porcelain sleeve, the stepscomprising wrapping with a mica tape that portion of the stud aboutwhich the porcelain sleeve is to be applied, the mica tape com rising afibrous sheet backing, a layer of mica flakes applied to the sheetbacking and a liquid binder uniting the mica flakes and the backing, themica tape wrapping being thick enough to nearly fill the space betweenthe sleeve the porcelain sleeve about attached to one end of the studfitting against one end of the porcelain sleeve to provide a liquidtight seal therewith and to position the porcelain sleeve, the assemblyof moved from the collar, heating the assembly to a temperature not inexcess of 175 C., evacuating the assembly, introducing into theevacuated annular cavity a liquid, completely reactive thermosettablecomposition comprising a monomeric compound having th group HzC=C thecomposition filling the cavity and impregnating the interstices of themica tape wrapping, and heating the applied composition to thermoset itto a solid substantially completely filling the annular space betweenthe porcelain sleeve and the stud, the porcelain sleeve, mica tape andthermoset com osition cooperating to produce the electrical insulationfor the bushing.

10. In the process of preparing a bushing comprising a stud and a boredporcelain sleeve shorter than the stud to be applied about the stud witha space between the stud and the bore of the porcelain sleeve, the stepscomprising wrapping with a mica tape that portion of the stud aboutwhich the porcelain sleeve is to be applied, the mica tape comprising afibrous sheet backing, a layer of mica flakes applied to the sheetbacking and a liquid binder uniting the mica flakes and the backing, themica tape wrapping being thick enough to nearly fill the space betweenthe sleeve and the stud, placing the porcelain sleeve about the studwith a collar attached to one end 01' the stud fitting against one endof the porcelain sleeve to provid a liquid tight seal and to positionthe porcelain sleeve, the assembly of stud, collar and the porcelainsleeve forming an annular cavity within which is disposed the mica tapewrapping, the annular cavity having an opening at the end of theporcelain sleeve removed from the collar, applying a coating of areadily removable protective material to the portions of the assemblyother than the annular cavity to prevent adhesive of subsequentlyapplied resins to the protected portions, heating the assembly to atemperature not in excess of C., evacuating the assembly, immersing theassembly into a body of a liquid resinous composition, completelyfilling the annular space between the porcelain and the stud,polymerizing the resinous composition, and removing the readilyremovable material from the assembly, the porcelain sleeve, mica tapeand thermoset composition cooperating to produce the electricalinsulation for the bushing.

11. In the process of preparing a bushing comprising a stud and a boredporcelain sleeve shorter than the stud to be applied about the stud witha space between the stud and the bore of the porcelain sleeve, the stepscomprising wrapping with a mica tape that portion of the stud aboutwhich the porcelain sleeve is to be applied, the mica tape comprising afibrous sheet backing, a layer of mica flakes applied to the sheetbacking and a liquid binder uniting the mica flakes and the backing, themica tape wrapping being thick enough to nearly fill the space betweenthe sleeve and the stud, placing the porcelain sleeve about the studwith a collar attached to one end of the stud fitting against one end ofthe porcelain sleeve to provide a liquid tight seal and to position theporcelain sleeve, applying another collar member at the other end of thestud to provide a liquid tight seal against the other end of thporcelain sleeve, the assembly of collars, stud and cavity a liquid,completely reactive thermosettable composition comprising a monomericcompound having the group H2C=C the composition filling the cavity andimpregnating the interstices of the mica tape wrapping, and heating JOHNS. JOHNSON. CHARLES C. STERRETT. JOSEPHINE R. GARBER.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,161,326 Webb June 6, 1939 2,479,357 Hill et al Aug. 16, 19492,479,400 Pecoroni Aug. 16, 1949 2,550,453 Coggeshall Apr. 24, 1951FOREIGN PATENTS Number Country Date 126,472 Australia Jan. 22, 1948371,216 Great Britain Apr. 21, 1932 537,268 Great Britain June 16, 1941

1. A BUSHING COMPRISING, IN COMBINATION, A STUD, AN INSULATING PORCELAINSLEEVE OF A LESSER LENGTH THAN THE STUD AND DISPOSED ABOUT THE OUTSIDEOF THE STUD, A SPACE BEING PRESENT ALONG THE FULL LENGTH BETWEEN THEPORCELAIN SLEEVE AND THE STUD, A WRAPPING OF A TAPE APPLIED TO THE STUDTO SUBSTANTIALLY FILL THE SPACE FOR THE ENTIRE LENGTH OF THE PORCELAINSLEEVE, THE TAPE COMPRISING MICA FLAKES AND A SHEET BACKING, AND A BODYOF THERMOSET RESIN FILLING THE ENTIRE SPACE AND SOLIDLY IMPREGNATING THETAPE WHEREBY NO VOIDS ARE PRESENT BETWEEN THE STUD AND THE PORCELAINSLEEVE, THE RESIN COMPRISING ESSENTIALLY THE PRODUCT DERIVED